Machines such as excavators, dozers, loaders, motor graders, and other types of heavy equipment use one or more hydraulic actuators to move a work tool. These actuators are fluidly connected to a pump on the machine that provides pressurized fluid to chambers within the actuators. As the pressurized fluid moves into or through the chambers, the pressure of the fluid acts on hydraulic surfaces of the chambers to affect movement of the actuator and the connected work tool. In an open-loop hydraulic system, fluid discharged from the actuator is directed into a low-pressure sump, from which the pump draws fluid. In a closed-loop hydraulic system, fluid discharged from the actuator is directed back into the pump and immediately recycled.
One problem associated with these types of hydraulic systems involves efficiency. In particular, the fluid discharged from the actuator can still have an elevated pressure, which represents unused hydraulic energy. In some situations, for example during overrunning conditions, this fluid discharged from the actuator can actually have a higher pressure than fluid entering the actuator. Unless captured and reused, the energy contained in the discharging fluid may be wasted, thereby lowering an efficiency of the hydraulic system. The efficiency may be lowered even further when the fluid is discharged into a low-pressure sump, as is the situation with open-loop systems.
A problem associated primarily with closed-loop hydraulic systems involves the need for significant fluid makeup and relief capacity. Specifically, the respective rates of hydraulic fluid flow into and out of different chambers of an actuator during different movements may not be equal. For example, because of the location of a rod within a first chamber of a hydraulic cylinder, an associated piston assembly may have a reduced pressure area within the first chamber, as compared with a pressure area within an opposing second chamber that does not include the rod. Accordingly, during retraction of the hydraulic cylinder, more hydraulic fluid may be forced out of the second chamber than can be consumed by the first chamber and, during extension, more hydraulic fluid may be consumed by the second chamber than is forced out of the first chamber. To accommodate these differences in fluid flows, closed-loop hydraulic systems commonly include makeup and relief circuits that provide additional fluid to the system (e.g., to the second chamber during extension) and/or consume excess fluid from the system (e.g., from the second chamber during retraction). These circuits, although imparting functionality to the associated systems, can increase cost and complexity of the system, while also consuming valuable space.
One method of improving the efficiency of a hydraulic system is described in U.S. Pat. No. 6,918,247 issued to Warner on Jul. 19, 2005 (the '247 patent). The '247 patent describes an open-loop hydraulic system having a pump configured to draw fluid from a low-pressure tank, pressurize the fluid, and direct the pressurized fluid into a boom actuator connected to pivot a boom of a machine. The system also includes an assist cylinder coupled to the boom of the machine, and an accumulator connected to one chamber of the assist cylinder. During movements of the boom from a high-potential energy position to a low-potential energy position (e.g., during lowering of the boom), gas within the assist cylinder is compressed. During a subsequent movement to the boom from the low potential energy position to the high potential energy position, the previously compressed gas is then allowed to expand and assist movement of the boom, thereby lowering an amount of energy required by the boom actuator to lift the boom.
Although the system of the '247 patent may help to improve efficiency through energy recuperation during an overrunning condition, it may still be less than optimal. In particular, the system may still be an open-loop system having associated throttling losses. In addition, because the system utilizes two different media (hydraulic fluid and compressible gas), the system may be overly complex and care should be taken to avoid cross-contamination of the media. Further, the system of the '247 patent may have no effect on the need for makeup or relief capacity in a closed-loop system.
The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art.